1. Field of the Invention
This invention relates to a focus detecting apparatus, and in particular to an apparatus for detecting the focus adjusted state of an objective lens.
2. Related Background Art
It is very common that a photographic camera or a video camera contains therein a detecting apparatus for automatic focus adjustment. However, it is at the center of the view finder that the distance measuring range is determined, and a camera in which the focus is adjusted to an object at a desired position in the picture plane of the finder has not yet been realized.
That is, when the focus detecting area is set at the center of the picture plane, there is no problem if the major portion of an object lies at the center of the picture plane, but if the major portion of the object is off the center of the picture plane, there is the inconvenience that erroneous distance measurement is caused and results in a blurred photograph. In order to eliminate such difficulty, the camera is once moved laterally and distance measurement is effected with the major portion of the object placed at the center of the picture plane, and the direction of the camera is returned to its original position while the then focus adjusted state is kept, whereafter the operation of releasing the shutter is performed. However, such operation is cumbersome, and cannot be executed in an urgent case. For example, it has been difficult to meet the requirement of taking a photograph in which a laterally moving object lies at a position off the center of the picture plane.
On the other hand, well known in an apparatus in which the imaging light beam received by an objective lens is directed to a set of re-imaging lenses and distributions of light quantity formed by these lenses are received by the picture element rows of a photoelectric converting element and the focus adjusted state of the objective lens is detected from the interval between the distributions of light quantity.
As a method for meeting the above-described requirement of effecting distance measurement without moving the camera with respect to the object lying at a position off the center of the picture plane, it is conceivable to dispose another picture element row outside the picture element row disposed on the optic axis and cause the newly provided picture element row to receive an object image off the center of the picture plane.
Japanese Laid-Open Patent Application No. 62-47612 and Japanese Laid-Open Patent Application No. 62-189415 disclose a focus detecting apparatus having a number of focus detecting areas. Also, for example, in U.S. Pat. No. 4,636,627, there is described an apparatus provided with a plurality of field controlling apertures.
Generally, in an optical lens system, aberrations are corrected well, but yet aberrations remain and the focus detecting area on or near the optic axis of the objective lens and the focus detecting area far from the optic axis differ greatly in the imaged condition on the picture element row as viewed, with the required performance as the standard. Accordingly, in the focus detecting area far from the optic axis, detection as accurate as that in the focus detecting area near to the optic axis cannot be realized, and this leads to the difficulty that highly accurate detection is limited to the focus detecting area near to the optic axis.